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Tadamasa was born in Osaka as the second son of Yūki Hideyasu.[1] His childhood name was Toramatsu (虎松) later become Toranosuke (虎之助). In 1607, he was received in audience by his grandfather, Tokugawa Ieyasu and uncle Tokugawa Hidetada. Hidetada took a liking to the boy, and ordered that he be raised in the Tokugawa household by Eishō-in together with Tokugawa Yorinobu. In the same year, he was assigned a fief of 10,000 koku, and became daimyō of Kazusa-Anegasaki Domain.

He was noted for his skill in the martial arts, and accompanied Hidetada during the Siege of Osaka, where he was frustrated that he would not be allowed to participate in the battle due to his youth. He strongly petitioned Hidetada to perform his genpuku ceremony before the start of the Osaka summer campaign, and Hidetada agreed, granting him a kanji from his name and Court rank of Senior Fifth Rank, Lower Grade and the courtesy title was Iyo-no-kami. He subsequently distinguished himself in combat with his prowess with the spear, which later became an heirloom of the Echizen-Matsudaira clan.

In 1637, he was disappointed that no order came to lead his troops during the Shimabara Rebellion, so he visited the battle in a private capacity with only twelve retainers. In 1643, he ordered the rebuilding of Mikuni Harbor as the main port of Fukui Domain. He died in 1648 at the domain's residence in Edo. On his death, seven of his senior retainers also committed junshi. His grave is at the temple of Eihei-ji in Fukui.